YQRES-03547; No. of pages: 11; 4C: 7
Quaternary Research xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Quaternary Research
journal homepage: www.elsevier.com/locate/yqres
High-elevation paleoenvironmental change during MIS 6–4 in the central
Rockies of Colorado as determined from pollen analysis
R. Scott Anderson a,b,⁎, Gonzalo Jiménez-Moreno c, Thomas Ager d, David Porinchu e
a
School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011, USA
Bilby Research Center, Northern Arizona University, Flagstaff, AZ 86011, USA
c
Departamento de Estratigrafía y Paleontología, Universidad de Granada, Fuente Nueva s/n, 18002 Granada, Spain
d
US Geological Survey, Mail Stop 980, Box 25046, Denver Federal Center, Denver, CO 80225, USA
e
Department of Geography, University of Georgia, 210 Field Street, Room 204, Athens, GA 30602, USA
b
a r t i c l e
i n f o
Article history:
Received 4 October 2013
Received in revised form 24 March 2014
Accepted 26 March 2014
Available online xxxx
Keywords:
Ziegler Reservoir
Interglaciations
Pollen analysis
High elevation
Climate variability
a b s t r a c t
Paleoecological studies from Rocky Mountain high elevations encompassing the previous interglacial (MIS 5e)
are rare. The ~10-m composite profile from the Ziegler Reservoir site (2705 m asl) of central Colorado allows
us to determine paleoenvironments from MIS 6–MIS 4 using pollen assemblages that are approximately
equivalent to marine oxygen isotope stages. During Pollen Zone (PZ) 6 time, pollen assemblages dominated by
Artemisia (sagebrush) suggest that alpine tundra or steppe occurred nearby. The transition to PZ 5e was characterized by a rapid increase in tree pollen, initially Picea (spruce) and Pinus (pine) but also Quercus (oak) and
Pseudotsuga menziesii (Douglas-fir). Non-arboreal pollen (NAP) types increased during PZ 5d, while Abies (fir)
and Juniperus (juniper) increased during PZ 5c. Pollen evidence suggests that temperatures during PZ 5b were
as cold as during PZ 6, with the site again surrounded by alpine tundra. Picea dominated during PZ 5a before
the onset of cooler conditions during PZ 4. The MIS 6–MIS 5e transition here was similar to the MIS 2–MIS 1 transition at other Rocky Mountain sites. However, the Ziegler Reservoir pollen record contains evidence suggesting
unexpected climatic trends at this site, including a warmer-than-expected MIS 5d and cooler-than-expected
MIS 5b.
© 2014 University of Washington. Published by Elsevier Inc. All rights reserved.
Introduction
Our ability to determine the long-term record of vegetation and climate change in western North America has been enhanced vastly by
analysis of long pollen records from long-lived basins. Several records
have used low-resolution techniques to provide the broadscale outline
of vegetation change over long time scales, including from Owens
Lake, California (Litwin et al., 1997; ca. 800 ka) and from the Great
Salt Lake, Utah (Davis, 1998; Davis and Moutoux, 1998; N1.5 Ma). On
the other hand, numerous high-resolution records of vegetation and climate change from continuous sedimentary sequences have also been
published that encompass multiple glacial–interglacial time scales. To
date, these records have been spatially dispersed in western North
America, with most being located along or near the Pacific margin
(Fig. 1). For example, the Humptulips (Heusser and Heusser, 1990)
and Carp Lake (Whitlock et al., 2000; marine oxygen isotope stages
(MIS) 1–5) sites are located in Washington state, while Clear Lake
⁎ Corresponding author.
E-mail address: [email protected] (R.S. Anderson).
(Adam and West, 1983; MIS 1-5), the Santa Barbara Basin (Heusser,
1995, 1998, 2000; MIS 1–6 +) and a recent Owens Lake record
(Woolfenden, 2003; MIS 1–5) are from California. More recently
Jiménez-Moreno et al. (2007) documented multiple glacial–interglacial
cycles (MIS 2–7) from Bear Lake, in the northern Great Basin along the
Utah–Idaho border. Only a single site has been studied from the Rocky
Mountain region. This is the record from the Valles Caldera of northern
New Mexico (Sears and Clisby, 1952; Fawcett et al., 2011), which encompasses MIS 10–14. These studies have become critical to our understanding of millennial-scale coupling of climate and vegetation change
and the long-term history of drought. However, with the exception of
the Valles Caldera study, previously compiled records have been from
low-elevation sites or have characterized the pollen record from very
large drainage basins.
Within the high altitudes of the central Rockies, paleoecological sites
extending backward in time beyond the Last Glacial Maximum (LGM)
are extremely rare, due largely to widespread ice that covered the
region during the Pinedale Glaciation (Pierce, 2004). A short list of
sites includes a sequence of buried lake sediments from Yellowstone
National Park, Wyoming (Baker and Richmond, 1978; Baker, 1986);
Devlins Park, Colorado (Legg and Baker, 1980); and the Mary Jane site
http://dx.doi.org/10.1016/j.yqres.2014.03.005
0033-5894/© 2014 University of Washington. Published by Elsevier Inc. All rights reserved.
Please cite this article as: Anderson, R.S., et al., High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as
determined from pollen analysis, Quaternary Research (2014), http://dx.doi.org/10.1016/j.yqres.2014.03.005
2
R.S. Anderson et al. / Quaternary Research xxx (2014) xxx–xxx
Figure 1. Location map of the Ziegler Reservoir fossil site in central Colorado. The site sits on a ridge between Snowmass and Brush Creeks. Other sites mentioned in the text include
(2) Humptulips, WA (Heusser and Heusser, 1990); (3) Carp Lake, WA (Whitlock et al., 2000); (4) Yellowstone sections (Baker and Richmond, 1978; Baker, 1986); (5) Bear Lake, UT/ID
(Jiménez-Moreno et al., 2007); (6) Great Salt Lake, UT (Davis, 1998; Davis and Moutoux, 1998); (7) Owens Lake (Litwin et al., 1997; Woolfenden, 2003); (8) Santa Barbara Basin, CA
(Heusser, 1995, 1998, 2000); (9) Valles Caldera, NM (Fawcett et al., 2011); (10) Tiago Lake, CO (Jiménez-Moreno et al., 2011); (11) Devlins Park site, CO (Legg and Baker, 1980);
(12) Mary Jane site, CO (Nelson et al., 1979); (13) Kite Lake, CO (Jiménez-Moreno and Anderson, 2012).
in Fraser Valley, Colorado (Nelson et al., 1979). However, the Mary Jane
and Devlins Park sites are limited to records of short duration not covering more than a few thousand years.
In October, 2010, while excavating the small ca. 5 ha Ziegler Reservoir basin near Snowmass Village, Colorado, in order to provide
additional water storage capacity, workmen uncovered rib bones, vertebrae and part of a tusk of a juvenile Columbian mammoth (Pigati et al.,
2014-in this volume). Since that time, the Ziegler Reservoir fossil site
has become one of the most prolific sources of bones and other paleontologic materials from any high-elevation paleoecologic site in the
Rocky Mountains (Johnson et al., 2014-in this volume). During the following field season, in May–June 2011, dozens of investigators converged on the site to collect samples for analysis of diverse proxies
from the exposed sediments that could be used to reconstruct the
paleoenvironments in the area when the small lake was in existence.
Our high-resolution pollen analysis of the nearly 10-m section of the
Ziegler Reservoir site exposed during the excavations is unique in that
it collectively spans a time range that we regard as equivalent to MIS
6 through MIS 4 in the mountainous terrain of central Colorado. In
this publication we describe the pollen assemblages and provide
paleoenvironmental and paleoclimatic interpretations.
Site setting — geology and chronology
The Ziegler Reservoir fossil site is a small basin ca. 300 m in diameter,
located at ca. 2705 m elevation within the Elk Mountains of westcentral Colorado (Fig. 1). Sediments were deposited at this site during
recession of the Bull Lake glaciation, during MIS 6 (Pigati et al., 2014in this volume). The basin sits within a set of nested lateral moraines,
deposited by at least one advance of the Snowmass Creek glacier
down modern Snowmass Creek Valley that was large enough to overtop
the eastern valley wall. This glacier originated in cirques high within the
Elk Mountains, being nearly 26 km in length and more than 250 m thick,
and terminating at 2315 m elevation (Pigati et al., 2014-in this volume).
When the ice retreated, a small basin was formed on top of the ridge between Snowmass Creek and the Brush Creek Valley to the east, which
became a small lake, surviving through the subsequent interglacial
(MIS 5) and into the beginning of the most recent glacial (MIS 4). Eventually the basin filled with at least 10 m of primarily organic-rich silt/
clays and peats above the till basement, transforming first from a lake
to a marsh or wetland, and later to an alpine meadow.
Bedrock of the site is largely Paleozoic sand-, silt- and mudstones
and shales, with Oligocene intrusives (Ogden, 1979; Pigati et al.,
Please cite this article as: Anderson, R.S., et al., High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as
determined from pollen analysis, Quaternary Research (2014), http://dx.doi.org/10.1016/j.yqres.2014.03.005
R.S. Anderson et al. / Quaternary Research xxx (2014) xxx–xxx
2014-in this volume). The late Cretaceous Mancos Shale underlies the
Quaternary sediments of the Ziegler Reservoir site, while clasts within
the enclosing Bull Lake moraines consist in large part of rocks of the
Pennsylvanian-Permian Maroon Formation, which outcrops at high elevation as peaks of the Maroon Bells to the south.
The chronology for the Ziegler fossil site record consists of a combination of 14C dates on lake sediment organics, a single 10Be date on a
boulder located on the enclosing moraine crest, and a series of optically
stimulated luminescence (OSL) ages (n=18) from different levels within the stratigraphic profile (Mahan et al., 2014-in this volume). OSL ages
occurred in stratigraphic order, were based on dose rates that exhibited
minimal scatter, and are replicable based on ages obtained from multiple samples from units 10 and 18.
Site setting — modern vegetation and climate
The modern vegetation of the southern Rockies of central Colorado
consists of alpine and upland herbaceous plant communities above ca.
3500 m asl, with high-elevation conifer forest and Populus tremuloides
(quaking aspen) above ca. 2900 m. Artemisia (Seriphidium; sagebrush)
steppe often occurs below this (RSA personal observations; Langenheim,
1962; Jiménez-Moreno and Anderson, 2012). The Picea engelmannii
(Engelmann spruce)–Abies lasiocarpa (subalpine fir) forest generally occurs over the range of ca. 2590 m to ca. 3800 m, but at the highest elevations occurs only as krummholz. It is best developed between ca. 3200 and
3500 m (Langenheim, 1962). Dominant trees include P. engelmannii and
A. lasiocarpa, but also P. tremuloides and Pinus contorta (lodgepole pine)
can be found. P. tremuloides often dominates with mixed conifers between
ca. 2900 m and 3200 m. Below ca. 2600 m, especially on dry sites or lower
valleys is the Artemisia steppe community. This community often includes
extensive stands of Quercus gambelii (scrub oak), as well as Chrysothamnus
spp. (rabbitbrush) and many other species.
Modern vegetation around the site is dominated by A. lasiocarpa and
P. tremuloides, with minor amounts of Q. gambelii and Artemisia
(Seriphidium) cf. arbusculum in forest openings. Small populations of
Pseudotsuga menziesii (Douglas-fir) and P. contorta also occur locally.
Additional shrubs include Symphoricarpos rotundifolius (snowberry),
Padus (Prunus) virginiana (chokecherry) and Rosa woodsii (rose).
Herbs include Fragaria cf. virginiana (strawberry), Heracleum
spondylium (cow parsnip), Lathyrus sp. (pea vine), Dephinium sp. (larkspur), Geranium richardsonii (wild geranium), Taraxacum officinale
(common dandelion), Lupinus sp. (lupine), Thalictrum sparsiflorum
(meadowrue), Verbascum thapsis (mullein), Achillea lanulosa (yarrow),
Bistorta bistortoides (bistort), Maianthemum (Smilacina) stellatum
(false Solomon's seal), Corallorhiza cf. trifida (coralroot), Pteridium
Aspen, Colorado
Avg Max Temp (C - blue)
Avg Precip (mm - red)
70
60
50
40
30
20
10
0
3
aquilinum (bracken), and members of Asteraceae, Apiaceae, Lamiaceae
and Violaceae. Terminology follows Weber and Wittmann (1996).
The climate of central Colorado is continental, with cold snowy
winters and cool moist summers. Winter precipitation arrives predominantly from Pacific frontal storms, while summer precipitation is
primarily from convective thunderstorms. Long-term climate data for
Aspen, Colorado (Aspen 1 SW; 39°11′ N, 106° 50′ W; 2487 m; data for
1980 to 2013 CE) ca. 13 km east–southeast of the Ziegler Reservoir
site show typical patterns for the central Rocky Mountains (Fig. 2).
Average monthly temperature varies from winter lows (DJF; − 12.8
to − 11.3°C) to summer highs (JJA; 22.6–25.6°C) (Western Regional
Climate Center, http://www.wrcc.dri.edu, accessed May 2013). Precipitation can occur in any month, with an annual average of 623.8 mm.
Average monthly precipitation varies from a high of 65.8 mm in March
and April to a minimum of 33.8 mm in June. Measurable snow accumulation has occurred in all months except JJAS (average annual =
2748 mm). Most snow falls during JFM (533, 711, 660 mm, respectively).
Methods
Samples for pollen analysis were collected in June 2011 from several
exposures that had been excavated by mechanized excavators, as well
as from a sediment core, with pollen samples coming from a number
of identified units (see stratigraphy in Pigati et al., 2014-in this
volume). Previous to our pollen sampling a total of 18 stratigraphic
units were identified in these exposures that could be traced laterally
across the basin. Excavation localities included a basinward trench — Locality 51, including units 18 to 15 — as well as a mid-basin exposure —
Locality 43, including units 14 to 8. Five sediment cores were drilled to
till using a Giddings Soil Probe at Locality 43, which recovered additional units (Unit 7 to basement till). Stratigraphic units are traced easily
across the basin but are best exposed where sampled. Because of this
the stratigraphic analyses reported here comprise a composite of
these units for the two localities. At least 525 bulk sediment samples
were collected for pollen at 1- or 2-cm depth intervals throughout the
stratigraphy, placed in whirlbags, transported back to the Laboratory
of Paleoecology (LOP) at Northern Arizona University (NAU), and
placed in cold storage.
In the laboratory 1 cm3 of sediment was subsampled from each of
the bags. Two Lycopodium tracer tablets were added to each of the samples for pollen concentration calculation. Pollen extraction followed a
modified Faegri and Iversen (1989) methodology, including suspension
in sodium pyrophosphate (when necessary) for disaggregation of clays,
sieving through 8-μm screen, and successive suspension in dilute KOH,
HCl, HF and acetolysis solution. The resulting residue was stained with
safranin-O and mounted in glycerol. Counting was performed at 400×
on a compound light microscope at the University of Granada (UGR).
Pollen was identified through comparison to the LOP and personal
reference collections, as well as using published keys. Percentages
were calculated and plotted as a composited diagram.
We also collected modern pollen samples both from moss polsters
around the Ziegler Reservoir site and from modern high-elevation
lake-core tops from 41 sites within 60 km of the Snowmastodon site.
These lakes ranged from 2869 m elevation in the Picea – mixed conifer
zone to above treeline at 3893 m. These pollen samples were processed
in the same manner as the fossil samples except without the sodium
pyrosphosphate treatment and counted at 400× at NAU.
Results
1
2
3
4
5
6
7
8
9
10
11
12
Month
Figure 2. Climate data (average maximum temperature and precipitation) for Aspen,
Colorado. The Aspen 1 SW recording station (2487 m) is ca. 13 km east–southeast of
Snowmass Village and the Ziegler Reservoir fossil site. Data accessed from Western Regional Climate Center, 23 May 2013.
Modern pollen assemblages
The vegetation directly around Ziegler Reservoir today is dominated
by P. tremuloides and A. lasiocarpa (see above), with only minor amounts
of Pinus trees and Quercus and Artemisia shrubs. However, the modern
pollen assemblage reflects the relative differences in pollen productivity
Please cite this article as: Anderson, R.S., et al., High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as
determined from pollen analysis, Quaternary Research (2014), http://dx.doi.org/10.1016/j.yqres.2014.03.005
4
R.S. Anderson et al. / Quaternary Research xxx (2014) xxx–xxx
Table 1
Modern lakes that provided core tops used in the modern pollen calibration.
Lake name
Code
Elevation (m)
Latitude (°N)
Longitude (°E)
Depth (m)
American
Anderson
Bear
Brady
Cathedral
Cleveland
Constantine
Crater
Diemer
Eagle
Galena South
Grizzly
Half Moon North
Half Moon South
Hard Scrabble
Hunky Dory
Independence
Linkin
Lost Man
Maroon
Missouri Adjacent
Missouri Central
Missouri North
Missouri South
Native
Petroleum
Seller
Seven Sisters Central
Seven Sisters North
Seven Sisters South
Seven Sisters West
Sopris
St. Kevin
Tabor Creek
Thomas North
Thomas South
Timberline
Tuhare East
Tuhare West
Weller
Whitney
AML
AND
BRL
BRD
CAT
CVL
CNS
CRL
DMR
EGL
GAL-S
GRZ
HFM-N
HFM-S
HRD
HDY
IND
LNK
LMN
MRL
MLA
MLC
MLN
MLS
NTV
PTL
SLR
SSC
SSN
SSS
SSW
SOP
SKL
TCL
TL-N
TL-S
TMB
TLE
TLW
WLL
WHT
3450
3584
3351
3353
3598
3609
3472
3053
2869
3074
3364
3788
3705
3648
3070
3452
3785
3639
3775
2903
3524
3488
3513
3477
3403
3729
3119
3755
3612
3708
3893
3364
3580
3588
3089
3114
3275
3691
3775
2894
3321
39.056434
39.020381
39.296418
39.368328
39.027824
39.421108
39.450279
39.085133
39.334721
40.210831
39.296689
39.050385
39.181211
39.178212
39.231250
39.421669
39.143998
39.128484
39.153144
39.097028
39.399236
39.396400
39.396400
39.387139
39.225326
39.026322
39.323608
39.441256
39.443272
39.436539
39.431906
39.371109
39.310593
39.053913
39.272701
39.269965
39.298019
39.448739
39.450614
39.115076
39.426186
−106.830143
−106.627456
−106.415324
−106.500557
−106.842861
−106.490829
−106.455002
−106.967266
−106.606941
−105.650284
−106.420133
−106.594205
−106.496153
−106.492944
−107.100606
−106.483612
−106.567429
−106.588388
−106.568847
−106.945398
−106.515367
−106.515328
−106.511150
−106.515500
−106.459205
−106.636675
−106.584717
−106.481147
−106.487361
−106.483056
−106.487269
−106.502502
−106.426860
−106.647472
−107.143641
−107.140331
−106.475281
−106.470111
−106.478136
−106.720923
−106.450547
10.5
3.5
4.6
2.1
6.7
6.7
3.7
3.3
2.6
2.0
3.1
13.7
5.3
5.6
4.1
2.5
6.9
9.0
16.7
3.1
2.5
3.2
5.9
3.6
0.9
20.0
3.6
5.8
5.6
1.2
6.9
5.4
6.8
24.0
9.5
5.7
10.4
11.0
25.5
8.7
7.8
and dispersal in alpine systems. It is dominated by Pinus (24.8%; mostly
P. contorta-type), Abies (23.8%), Picea (10%), Quercus (10.3%), Artemisia
(14.5%), with only minor amounts of P. tremuloides (1.6%), P. menziesii
(0.3%), Poaceae (1.9%) and Atriplex-type (1.9%). Other herbs total 6.6%
of the sum.
Important characteristics of the high-elevation lakes comprising the
modern pollen dataset for this study are shown in Table 1. While changes in many pollen species occur with elevation, others – such as
Juniperus, Artemisia, Ambrosia, Sarcobatus, Asteraceae and Poaceae –
show little variability (Fig. 3). In general, herbaceous pollen types are
more abundant within the Picea–Abies–P. contorta zone and above
(Fig. 3).
Nine core top pollen samples (2869–3119 m) come from the Picea–
Pinus–mixed conifer forest, with dominant species P. engelmannii,
P. tremuloides, Salix and Alnus, with minor amounts of Picea pungens
(Colorado blue spruce), A. lasiocarpa or A. concolor (white fir),
P. contorta and numerous herbaceous species. Pollen assemblages
from these samples are dominated by Pinus (18–76%; median =
43.4), Picea (4–34%; median = 15.4), Quercus (2–14%; median =
5.7) and Amaranthaceae (cf. Atriplex, 1–9%, median = 3.9). Pollen of
P. menziesii (0–2.6%; median = 0.3) is primarily found in the zone, as
is Ephedra (0–1.4%; median = 0.6), Salix (0–2.7%; median = 1.3) and
Brassicaceae (0–1.7%; median = 0). P. contorta-type is at a minimum
at this elevation (0.7–10.6%; median = 3.8).
Fifteen samples (3275–3524 m) came from Picea–Abies–P. contorta
forest where P. engelmannii, P. contorta and P. tremuloides dominate
with a diverse herbaceous understory. Total Pinus (42–63%; median =
55.2) dominates samples from this elevational range. Picea remains important (6–30%; median = 11.4). P. contorta-type increases (2–10.5%;
median = 7.1), while Abies (0.3–5.2%; median = 1.2), Alnus (0–0.6%;
median = 0.3) and Populus (0–1.0%; median = 0.3) reach their highest
pollen percents in this group. Those pollen types that decline include
P. menziesii, Salix, Quercus, Ephedra and Amaranthaceae.
Ten samples come from the P. engelmannii-dominated krummholz
zone with elevations 3580 to 3705 m. Salix, Cyperaceae, Dryas and
other alpine herbs also occur here. Pinus continues to dominate
(51–71%; median = 59.3) with somewhat diminished Picea
(7.4–20.3%; median = 11.1), P. contorta (2.2–8.7%; median = 4.9),
Abies (0.3–1.0%; median = 0.6), Alnus (0-0.6%; median = 0.1) and
Quercus (0–3.3%; median = 1.1). High-elevation herbs continue to be
important in the pollen of this elevation range.
Lastly, eight samples were taken from lakes above treeline (3708–
3893 m) in the alpine zone where dwarf Salix, Cyperaceae, Dryas,
Pedicularis, Ranunculaceae and members of the Asteraceae dominated.
Again, Pinus continues to dominate (55–67%; median = 62.1), while
P. contorta-type increases slightly as well (4.6–15.8%; median = 8.7).
Picea pollen percents are comparable to the krummholz values. Abies
is reduced, while Quercus and Alnus dwindle to trace amounts. Herbaceous pollen is most important in this zone (Fig. 3).
Using the 41 pollen assemblages from the core tops we calculated a
ratio of spruce:pine (Picea:Pinus; S:P ratio) (Fig. 4). This ratio has been
used in several studies in the southern Rockies (Markgraf and Scott,
Please cite this article as: Anderson, R.S., et al., High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as
determined from pollen analysis, Quaternary Research (2014), http://dx.doi.org/10.1016/j.yqres.2014.03.005
5
R.S. Anderson et al. / Quaternary Research xxx (2014) xxx–xxx
Figure 3. Modern pollen surface samples from high-elevation lakes near the Ziegler Reservoir site. Lake tops were collected as part of the chironomid calibration network of Haskett and
Porinchu (2014-in this volume). Sites range from 2869 m in Picea–Pinus — mixed conifer forest to above treeline at 3893 m.
Stratigraphic pollen
Spruce/Pine Ratio
Based on the OSL chronology, we believe our pollen record essentially spans the lower part of MIS 4 time (older than ~ 70.2 ka) into the
upper part of MIS 6 time (to ~139.9 ka). However, we describe the pollen assemblages below as biostratigraphic zones rather than as time
stratigraphic zones. We believe that our pollen zones (PZ) correlate
strongly with the marine oxygen isotope stages, and we number them
in a similar fashion. However, the equivalence is not exact (Table 2),
with the offset in age increasing with increasing depth. Here, the similar
numbering schemes are for convenience of discussion. We analyzed 123
pollen samples in this ~69,700-yr record, corresponding to an average
sampling interval of ~ 570 yr. This relatively high-resolution record is
comparable to the Jiménez-Moreno et al. (2007) study from Bear Lake,
UT/ID, where the average sampling interval was ~ 600 yr. Individual
pollen zones are described below, in ascending stratigraphic order.
PZ 6 (1017–991 cm [dating ~140.4–137.7 ka])
Pollen analyses from this interval were from samples taken near the
bottom of the Giddings drill core ZR-3C (at Locality 43, including units 3
& 4), where sediment is a dark brown clay with fine silt, existing immediately above till (Pigati et al., 2014-in this volume). Pollen assemblages
in this group are dominated by non-arboreal (NAP) pollen, principally
by Artemisia (31–60%) (Fig. 5), with minor amounts of Amaranthaceae
(cf. Atriplex; 2–6%), Poaceae (grasses; 3–6%), and Asteraceae (4–6%). Conifer pollen is dominated by Pinus (16–23%), with the greatest proportion identifiable as high-elevation Pinus (P. cf. haploxylon) (Fig. 5).
Picea pollen is 3–9% of the sum, while Abies, Juniperus and Quercus are
all N1% of the sum.
1.8
1.8
1.6
1.6
1.4
1.4
1.2
1.0
Alpine
Spruce
Krummholz
0.8
Spruce –
Fir –
Lodgepole
1.2
Spruce PineMixed
conifer
0.6
1.0
0.8
0.6
0.4
0.4
0.2
0.2
0
4000
3800
3600
3400
3200
3000
2800
2600
Spruce/Pine Ratio
1981; Carrara et al., 1984; Fall, 1997; Reasoner and Jodry, 2000; Toney
and Anderson, 2006; Jiménez-Moreno et al., 2008, 2011) to track the
density of the Picea forest. For our modern pollen data, the ratio is
often near or above 0.4 within the Picea dominated forests below ca.
3500 m (Fig. 4). It is always below 0.3 within the Picea krummholz,
and below 0.2 in the alpine zone where no trees are present.
0
Elevation (m)
Figure 4. Ratio spruce to pine (Picea:Pinus; S:P) pollen percentages for the modern pollen surface samples shown in Fig. 3. Note that for those samples located within the modern Picea
krummholz zone the ratio is b0.3, and for those within the modern alpine zone it is b0.2.
Please cite this article as: Anderson, R.S., et al., High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as
determined from pollen analysis, Quaternary Research (2014), http://dx.doi.org/10.1016/j.yqres.2014.03.005
6
R.S. Anderson et al. / Quaternary Research xxx (2014) xxx–xxx
Table 2
Comparison of ages and uncertainties for the MIS 6/5 and 5/4 boundaries and MIS 5 substages from the SPECMAP and LR04 isotopic records (Martinson et al., 1987; Lisiecki and Raymo,
2005) with the pollen record from the Ziegler Reservoir site (this study; Mahan et al., 2014-in this volume).
MIS boundary
SPECMAP age, (ka)
Uncertainty, ±1σ (ka)
LR04 age, (ka)a
This study age (ka)b
Uncertainty, ±1σ (ka)
MIS 5/4
5a (peak)
5b (peak)
5c (peak)
5d (peak)
5e (peak)
MIS 6/5
73.9
79.2
90.0
99.3
110.7
123.8
129.8
2.5
3.5
6.8
3.4
6.2
2.6
3.0
71
82
87
96
109
123
130
77.0
83.1
94.4
103.5
117.7
133.2
137.7
3.4
4.0
3.9
5.1
8.4
11.3
13.1
a
Uncertainty in the LR04 age model is ±4 ka from 1 to 0 Ma (Lisiecki and Raymo, 2005).
Ages for the MIS 5 substages were defined as follows (Fig. 7): MIS 5e: maximum values in Quercus and Pseudotsuga. MIS 5d: minimum central value in arboreal pollen (AP); MIS 5c:
peak in AP; MIS 5b: peak central value in Artemisia. MIS 5a: peak in AP, minimum value in Artemisia. Even if our dates for the different climatic events seem to be older, they fall within the
age uncertainty of the different age models.
b
PZ 5e (991–905 cm [dating ~137.7–129.0 ka])
Pollen analyses from in this interval also came from samples taken in
the lower part of drill core ZR-3C (including units 4–6). Sediment is described as organic-rich silt with sparse carbonate lenses near the base,
grading upward into a laminated silty clay in the middle of the unit,
and overlain by an organic-rich silt with abundant plant remains (Pigati
et al., 2014-in this volume). Major changes in the pollen assemblages
occur in this zone, including a significant decline in Artemisia (~5–20%)
and a change to dominance by arboreal pollen (AP). Conifer pollen is
again dominated by Pinus (14–31%), of which the majority identified is
P. haploxylon-type. Picea increases substantially (up to 25% of the sum),
and Quercus also increases to a maximum of 21% (Fig. 5). Pollen of
P. menziesii (to 3%), Abies (generally b1%) and Juniperus (up to 3%) increases, as does pollen of Asteraceae (4–20%). Amaranthaceae (probably
Atriplex) increases to nearly 12% in the lower part of the interval, but declines in the upper part. Poaceae pollen declines, while pollen percentages
of Sarcobatus are largely unchanged from below. Cyperaceae increases to
over 7% of the pollen sum within this zone (Fig. 5).
PZ 5d (905–708 cm [dating ~129.0–113.0 ka])
Pollen analyses in PZ 5d were from samples taken from the top of drill
core ZR-3C (including units 6–8). Sediments in these units include
organic-rich silts at the base, grading into a sandy silt with scattered carbonate lenses, and finally into another organic-rich silt (Pigati et al., 2014in this volume). In PZ 5d, %AP declines such that the AP:NAP ratio becomes essentially 1:1. This increase in NAP is not due to increasing Artemisia, but instead due to significant increases in both Amaranthaceae
(Atriplex; 5–16%) and Asteraceae (11–21%) pollen (Fig. 5). Pinus and
P. haploxylon-type pollen are slightly diminished, but continue to dominate the conifers. However, Pinus percentages decline by almost half
from the base of the interval to its top. Picea pollen remains largely unchanged from PZ 5e below. Abies pollen increases (to nearly 6%), Juniperus
remains relatively unchanged, but Pseudotsuga declines to 1% or less
(Fig. 5). Within the hardwood pollen component, Quercus percentages remain relatively high at 8–19%. Cyperaceae percentages and the amount of
Pediastrum cell nets also increase relative to their previous abundances.
PZ 5c (708–548 cm [dating ~113.0–100.0 ka])
Sixteen pollen analyses in this zone were made from samples taken
from units 8–12 exposed in vertical section in the highwall at Locality 43
(Pigati et al., 2014-in this volume). Sedimentary textures within this
zone transition from organic silt in Unit 8 to mottled brown silt in
Unit 9, which is overlain by yellowish-brown bedded silt with abundant
fibrous organic remains, seeds, twigs and wood in Unit 10 (informally
referred to as the “yellow brick road”). These are overlain by a weakly
banded silt with organics in Unit 11, and finally an organic mat with remains of Cyperaceae leaves and wood in Unit 12. Pollen assemblages
from this period are characterized by a slight increase in the arboreal
pollen percentages, largely a result of increases in Abies (from 4% to
N20% at the top of the zone) and Juniperus (to 9% by mid zone, but declining upsection). This zone also includes low but consistent
occurrences of both Pinus edulis-type (Colorado piñon) and Cercocarpus
(mountain mahogany), as well as a slight increase in Poaceae. Several
pollen types yielded high percentages that declined upward through
the zone, including Pinus (16–8%), Quercus (14–3%), Asteraceae (19 to
ca. 10%) and Amaranthaceae (15–6%). Cyperaceae also declines. Artemisia percentage does not differ greatly from its occurrence in lower zones.
Peaks in Abies and Picea are associated with sediments of Unit 10.
Pediastrum cell nets are at their highest percentages during PZ 5c, and
Botryococcus increases substantially higher in the zone.
PZ 5b (548–346 cm [dating ~100.0–87.9 ka])
Nineteen pollen samples were analyzed from units 13 and 14 at Locality 43. These sediments are banded (dark yellowish brown and very
dark grayish brown) silty clays in Unit 13 overlain by massive grayish
brown silty clays in Unit 14 (Pigati et al., 2014-in this volume). Pollen
of PZ 5b is characterized by an abrupt and sustained increase in Artemisia pollen, from 36% to over 64%. This maximum abundance is similar to
Artemisia abundances observed in PZ 6. Amaranthaceae (from 6 to 14%),
Poaceae (5–11%) and Sarcobatus (2–5%) are other principle pollen types
that increase in this zone. All other pollen – Picea, Pseudotsuga, Abies,
Quercus, Juniperus and Asteraceae – decline. Pinus is highly variable,
with abundances exceeding 16% low in the zone. Percentages of colonial
algae – Pediastrum and Botryococcus – reach near maximum values.
PZ 5a (346–219 cm [dating ~87.9–77.3 ka])
Nineteen pollen samples were analyzed from units 15 and 16 at Locality 51. Sediment in Unit 15 is a peaty silt, while Unit 16 is a very dark
brown peat with little mineral matter (Pigati et al., 2014-in this
volume). The lower part of this zone shows slight increases in both
Quercus and Abies, but less than their relative abundances observed in
PZ 5c. Picea increases rapidly from b 1% to N 26% between two
stratigraphically adjacent samples; however while Picea increases in
abundance upward through the zone, both Quercus and Abies decline.
Pinus increases gradually from nearly 10–20% by zone's end. The only
other major pollen type to increase is Cyperaceae (Fig. 5). All other pollen types, especially Artemisia, decline during PZ 5a. This also includes
the limnic indicators, Pediastrum and Botryococcus.
PZ 4 (219–146 cm [dating ~77.3–at least 70.0 ka])
Only twelve pollen samples were taken and analyzed from units 17
and 18, owing to increasingly poor preservation upsection in Unit 18.
Samples from this zone were derived from Locality 51. Sediments consist
of a clayey silt in Unit 17, grading upward to a mottled light gray clay in
Unit 18 (Pigati et al., 2014-in this volume). Pollen of this zone shows increases in Pinus and NAP (non-arboreal pollen), especially Artemisia,
and Cyperaceae reaches its maximum observed abundance in this entire
record (Fig. 5). Picea percentages decline upsection in this zone precipitously while other conifers (Abies, Juniperus, Pseudotsuga) and
Please cite this article as: Anderson, R.S., et al., High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as
determined from pollen analysis, Quaternary Research (2014), http://dx.doi.org/10.1016/j.yqres.2014.03.005
7
Figure 5. Summary of major stratigraphic pollen percentage assemblages from the Ziegler Reservoir site, Colorado. Arboreal pollen shown in green curves; non-arboreal pollen shown in yellow; aquatic and wetland pollen and spores shown in blue.
R.S. Anderson et al. / Quaternary Research xxx (2014) xxx–xxx
Please cite this article as: Anderson, R.S., et al., High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as
determined from pollen analysis, Quaternary Research (2014), http://dx.doi.org/10.1016/j.yqres.2014.03.005
8
R.S. Anderson et al. / Quaternary Research xxx (2014) xxx–xxx
hardwoods (Quercus) exhibit minimal occurrences. Shrubby taxa, including Sarcobatus, Asteraceae and Amaranthaceae, along with Poaceae, also
approach minimum values for the entire record.
Discussion
Analyses of sediments at the Ziegler Reservoir fossil site from samples
taken from a series of cut faces, pits and drill cores have provided a unique
opportunity to investigate the history of vegetation changes from a highelevation location during what appears to be the transition from the previous glacial (Bull Lake, MIS 6) through the penultimate interglacial (Sangamon, MIS 5e) and into the beginning of the most recent glacial (Early
Wisconsin, MIS 5d –4). To our knowledge, this ~2705 m elevation site
is the highest-altitude locality in western North America to provide a
high-resolution record of vegetation change for this time period. The
chronology consists primarily of OSL ages (Mahan et al., 2014-in this
volume). This chronology has also allowed us to examine the structure
of the previous interglacial independent of global curves such as
Martinson et al. (1987). However, our dating scheme tends to systematically over-estimate the generally accepted age boundaries, with increasing departure from Martinson et al. (1987) with depth (Table 2).
Deposition at the site appears to have begun at the end of the Bull
Lake glaciation (Bryant and Martin, 1988). One lateral moraine formed
the north, east and south boundaries of the basin, while a second
formed the west boundary (Pigati et al., 2014-in this volume). When
the ice retreated it left a small, enclosed basin that was not overridden
by glaciers during the Pinedale glaciation. A limited drainage basin
with no inflowing streams and slow input of eolian sediments allowed
the lake to exist for up to 70,000 yr before infilling arrested any significant amount of subsequent deposition.
Today the site is surrounded by conifer–Populus forest. Interpretation of the pollen from Ziegler Reservoir is facilitated by comparison to
two modern pollen datasets, including 41 modern core-top samples
from lakes at 2869 to 3893 m elevation (this study), and 46 core-top
and polster samples from 2470 to 3780 m elevation near Cottonwood
Pass to the south (Fall, 1992), and two nearby high-elevation pollen records. Pollen from the bottommost samples of the record is dominated
by non-arboreal pollen, predominantly Artemisia. This suggests
steppe-like vegetation, where trees such as Picea and even Pinus may
have been sparse to absent. During that time, which we interpret to
be temporally equivalent to latest MIS 6, Artemisia attains 60%. No modern pollen samples in either this study or in the Fall (1992) datasets
have such high Artemisia percentages, although the lowest elevation
Artemisia steppe samples of Fall (1992) approach this. The closest fossil
analogs to this come from the Last Glacial Maximum (LGM; MIS 2) pollen spectra of Tiago Lake (2700 m elevation; ca. 155 km north) and Kite
Lake (3665 m elevation, ca. 70 km east). In both cases Artemisia percentages reach 40–60% of the sum (Jiménez-Moreno et al., 2011;
Jiménez-Moreno and Anderson, 2012) and were interpreted as representative of alpine tundra. Such an interpretation also is supported by
the low S:P ratio, where ratios of b 0.3 suggest Picea krummholz ecotype
(Fig. 6).
The transition out of MIS 6 time about 137 ka into the MIS 5 time interglacial showed a rapid increase in tree species pollen during PZ-5e,
manifested first by Picea and then by Pinus (Fig. 5). This is closely
analogous to the late-glacial – early Holocene vegetative transition at
Tiago Lake – and to a lesser extent the vegetative transition observed
in sediments at the higher elevation Kite Lake. The S:P ratio during
this interval at Ziegler Reservoir suggests the presence of a moderately
dense Picea forest (Fig. 6). Although the pollen records for Tiago and
Kite Lakes exhibit millennial-scale abrupt warmings and coolings that
have been interpreted to correlate directly to the Bølling - Allerød and
Younger Dryas oscillations, no analogous warm-to-cold-to-warm
millennial-scale deglaciation pattern could be confirmed in our PZ
6–PZ 5e transition at Ziegler Reservoir.
The major difference between our interpreted PZ-5e data and those
from MIS 1 at Tiago and Kite Lake is in the importance of P. menziesii
here, but especially in Quercus during this period. Quercus is most abundant at Ziegler during the early interglacial, peaking in the middle of
PZ-5e time. Although Quercus is most abundant during the midHolocene at Tiago Lake it never exceeds ca. 10% (Jiménez-Moreno and
Anderson, 2012) compared to 21% in the Ziegler record. Quercus is
never significant at the much higher Kite Lake site (Jiménez-Moreno
et al., 2011). Similarly, although a few modern samples from the
Picea–mixed conifer zone (Fig. 3) exceed 10%, Quercus never exceeds
5% of the pollen sum in the Fall (1992) dataset. This suggests the importance of Quercus in the local vegetation around the Ziegler Reservoir site
during the last interglacial, a situation that may have no known analog
for this elevation today, and supports the contention that MIS 5e was
warmer than the present interglacial (Kukla et al., 2002; CAPE Last Interglacial Project Members, 2006; Jiménez-Moreno et al., 2007).
The transition to MIS 5d, here dated to about 129 ka, is suggested by
subtle changes in the pollen record, with increasing non-arboreal pollen
types, specifically in Amaranthaceae (probably Atriplex) and Asteraceae.
The S:P ratio suggests initially the forest was sparser than during PZ-5e
time (Fig. 6). The fact that Quercus pollen percentages declined little –
only to ~ 10% or so – suggests to us that temperatures were probably
only slightly cooler than in PZ 5e. These pollen assemblages are very
similar to modern pollen from the Picea–mixed conifer forest of today
(Fig. 3).
Pollen spectra from PZ 5c time – beginning ~ 113 ka – show an increasingly dense Picea forest (increasing S:P ratios; Fig. 6), suggesting
Figure 6. Pollen ratio (S:P) of Picea to Pinus percentages for the Ziegler Reservoir site fossil pollen assemblages. Based on the relationships shown in Fig. 4, the ratios suggest that during PZ 6
time the site was near treeline, and within PZ 5b time it might have been above treeline.
Please cite this article as: Anderson, R.S., et al., High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as
determined from pollen analysis, Quaternary Research (2014), http://dx.doi.org/10.1016/j.yqres.2014.03.005
R.S. Anderson et al. / Quaternary Research xxx (2014) xxx–xxx
a more boreal-type (Picea dominant) vegetation community. This shift
would be consistent with declines in Quercus, Asteraceae and cf. Atriplex,
presumed to grow lower in elevation. However, the ratio is equally influenced by increases in Picea or by declines in Pinus (Fig. 5), the latter
of which appears to have been replaced around the Ziegler Reservoir
site by Abies near the top of the zone. In both the Fall (1992) modern
pollen dataset as well as in our own (Fig. 3), Abies is more abundant in
the Picea – Abies – P. contorta forest. In no case does Abies ever exceed
10% of the pollen sum in these modern samples. In contrast, during
the temporal equivalent of MIS 5c Abies comprises 15–30% of the sum
(Fig. 5). This suggests that Abies was a major component of the forest
around the lake between ca. 105 and 100 ka, much like it is today,
where it is the dominant conifer in the forest and produces nearly 24%
of the modern pollen from a polster collected around the lake. High percentages of the algal species Pediastrum and Botryococcus (Fig. 5) suggest that either the lake was dessicating during this time or that a
threshold was achieved in which the lake was infilling at an increasingly
9
rapid rate. A slight increase in the sediment accumulation rate (SAR)
during this time supports the latter explanation.
The pollen spectra from PZ-5b, beginning by ~100 ka, suggest very
cool and/or dry conditions prevailed at the site for nearly 12,000 yr. Pollen of Artemisia once again dominates the assemblages, and the overall
spectra are nearly identical to those from PZ 6 (Fig. 5). Increases in
Poaceae occur, as well as in Amaranthaceae (cf. Atriplex) and Sarcobatus.
These pollen types are most common at lower elevations today and
their deposition during this time suggests that the vegetation around
the Ziegler Reservoir site was quite open, a conclusion that is supported
by the S:P ratio as well (Fig. 6). The S:P ratio of fully half of the individual
pollen assemblages during this period is either below 0.3 (orange
shorter dashed line in Fig. 6; threshold for Picea krummholz vegetation)
or 0.2 (red longer dashed line in Fig. 6; threshold for alpine vegetation).
We interpret this period as being at or near treeline, which suggests a
lowering of treeline during PZ-5b of ca. 800–1000 m. Data from the
Ziegler site suggesting PZ 5b was as cool as PZ 6 was unanticipated.
Figure 7. Selected pollen percentages from the Ziegler Reservoir site in comparison to the summer insolation (Laskar et al., 2004) and SPECMAP and LR04 oxygen isotope curves
(Martinson et al., 1987; Lisiecki and Raymo, 2005). MIS stages are placed following the Martinson et al. (1987) nomenclature. Relatively warm and cold pollen zones are shown with
light red and blue shading, respectively. A tentative correlation between the maxima in summer insolation during MIS 5e and the pollen record (PZ 5e) is shown through a dashed line.
Please cite this article as: Anderson, R.S., et al., High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as
determined from pollen analysis, Quaternary Research (2014), http://dx.doi.org/10.1016/j.yqres.2014.03.005
10
R.S. Anderson et al. / Quaternary Research xxx (2014) xxx–xxx
The SPECMAP δ18O curve of Martinson et al. (1987) documents a progressively cooler MIS 5b than MIS 5d, with neither being as cold as
MIS 6 (Bull Lake) or MIS 2 (Pinedale). One possibility is that this
might reflect local conditions within the study area, perhaps particularly
dry conditions, which opened the forest more than any other time during MIS 5 time, and allowed for greater deposition of pollen from shrubby taxa, both locally (probably Artemisia, perhaps Poaceae) and from
lower elevations (Atriplex and Sarcobatus). This interpretation is supported in part by the abundance of the algae Botryococcus during this
time, which suggests low water levels in the lake.
The transition from PZ 5b to PZ 5a at ca. 87.9 ka is abrupt and dramatic, as arboreal pollen increases substantially, in Picea, Abies, Pinus
and Quercus (Fig. 5). We note that this transition also represents a
sub-basin site change in our record, from Locality 43 to 51. However,
we do not attribute the shift in observed pollen signal to the shift in locality (effectively placed here at 346 cm depth in our composite record)
because of the physical correlation of laterally pervasive sedimentological units. Such high percentages of Picea are characteristic of subalpine
forest (this study; Fall, 1992). The S:P ratio suggests a mostly closed
Picea (and Abies?) forest existed around the Ziegler Reservoir site. The
characteristics of the sediments (largely peats) and increases in
Cyperaceae pollen suggest that the lake had by this time transitioned
to a shallow-water marsh, and the high Picea percentages suggest that
trees may have begun to encroach onto the margins of the site. The
progressively smaller amounts of Quercus may be a result of the closed
nature of the forest, or due to generally cooler temperatures.
Only a few pollen samples in our dataset document the onset of PZ 4,
and they generally record a considerable increase in non-arboreal pollen, primarily Artemisia. The S:P ratio suggests much more open conditions than in PZ 5a, but not as open as in PZ 5b. Based on minimum
occurrence of other tree species – Abies, Pseudotsuga, Quercus –
combined with moderate abundances of Picea and Pinus, we suggest a
mosaic of subalpine Picea and Pinus with more abundant Artemisia, all
suggesting cooler and drier conditions. Cyperaceae pollen reaches its
abundance maximum of the entire composite record, and it is during
this time that we interpret the former lake to have completed its transition to a wet meadow.
age pollen assemblages are dominated by Artemisia and Poaceae, but by
the earliest part of the transition to the subsequent interglacial, Picea
and Pinus became the dominant vegetation in both cases. Other differences include the greater component of mixed conifer forest at Tiago
Lake (greater Pinus, where P. contorta prevails; Abies; P. menziesii) and
considerably greater importance of Q. gambelii at Ziegler. On the other
hand, both sites document early-to-middle interglacial increases in
Amaranthaceae and Juniperus, which we attribute to be derived from
pollen from the lower elevation Atriplex flats and Juniperus woodland.
The paleoenvironmental implications of both PZ 5d and 5b pollen
assemblages were unanticipated and counterintuitive. Although MIS
5d is generally considered to be cooler than MIS 5e, the arboreal pollen
record is little different between the two temporally analogous zones at
Ziegler Reservoir (Fig. 5). For example, although there is a slight increase in Pinus and Abies and a slight decline in Quercus, the primary
changes are in pollen types of lower elevation, e.g., Amaranthaceae
(cf. Atriplex) and Asteraceae (Fig. 5). This suggests an actual expansion
of Atriplex scrub, since the S:P ratio remains much the same as before
(Fig. 6), again, suggesting little change in the characteristics of the forest
at the Ziegler Reservoir site during that time. In contrast, PZ 5b appears
to be nearly as cold as latest PZ 6, but these changes might reflect local
drying during PZ 5b. In any event, temperature interpretations for both
PZ 5d and PZ 5b are somewhat at odds with relative trends in the δ18O
record from Greenland (Johnsen et al., 2001).
Although it is important to understand the late Pleistocene vegetation shifts suggested by the pollen assemblages from the Ziegler Reservoir site, pollen analysis remains only one of several proxy studies
relevant to reconstructing the former environments of this highelevation site (Johnson et al., in this volume). Studies of the proboscideans (Fisher et al., 2014-in this volume), plant macrofossils (Brown
et al., 2014-in this volume; Miller et al., 2014-in this volume;
Strickland et al., 2014-in this volume), insects (Elias, 2014-in this
volume), chironomids (Haskett and Porinchu, 2014-in this volume),
snails and ostracodes (Sharpe and Bright, 2014-in this volume) and
other organisms will be combined with the pollen data to provide a
more complete paleoenvironmental picture of the Ziegler Reservoir
site over its ~70,000-yr history.
Conclusions
Acknowledgments
Our analysis of the ~70 ka pollen record from the Ziegler Reservoir
site in central Colorado has allowed us to determine vegetation
sequences of glacial–interglacial–glacial transitions from a site with
great sensitivity to climate change. While several other sites have
documented characteristics of the previous interglacial, these have
come primarily from large lakes at lower elevations in western North
America (see compilation in Jiménez-Moreno et al., 2007). To our
knowledge the Ziegler Reservoir pollen record is unique in its length
and antiquity for high-elevation sites within the Rockies. In this regard,
it is important to examine as many records of interglacial conditions as
possible so that we are able to document the range of natural variability
inherent in local sequences while placing them into a hemispheric or
global context.
In our interpretations we have benefitted greatly from an internallyderived chronology for this depositional record (Mahan et al., 2014-in
this volume). This has allowed us to interpret vegetation change within
an independent temporal framework, which does appear in general accord with the SPECMAP chronology (Martinson et al., 1987) – but with
increasingly older ages with depth (Table 2) – and with variability in
summer seasonal solar insolation (Fig. 7). But there exist significant differences (below).
The transition in the early part of the record – essentially correlated
to MIS 6 to MIS 5e – is strikingly similar to the MIS 2–MIS 1 record from
Tiago Lake (Jiménez-Moreno et al., 2011), a lake presently at nearly the
same altitude (Ziegler Reservoir = 2705 m; Tiago Lake = 2700 m). Both
sites today occur in conifer–P. tremuloides forest. In both records, glacial-
We thank Kirk Johnson, Ian Miller and Jeff Pigati for bringing this important site to our attention. We also thank Damara Kautz, Spencer
Rubin and Martina Tingley for assistance in collecting the pollen
samples; Silke Buschmann for pollen processing; Monique Belanger
for data entry; and Monica Saaty for Figure 1. We greatly appreciated
comments on the manuscript by Wally Wolfenden, Alan Gillespie, Jeff
Pigati and an anonymous reviewer. Funding for this project came from
grants from the Denver Museum of Nature and Science (03-377 NAU2011 and 03-377 NAU-2012) and the U.S. Geological Survey's Climate
and Land Use Change Research and Development Program
(G13AC00022).
References
Adam, D.P., West, J.G., 1983. Temperature and precipitation estimates through the last
glacial cycle from Clear Lake, California, pollen data. Science 219 (4581), 168–170.
Baker, R.G., 1986. Sangamonian (?) and Wisconsinan paleoenvironments in Yellowstone
National Park. Geological Society of America Bulletin 97, 717–736.
Baker, R.G., Richmond, G.M., 1978. Geology, palynology and climatic significance of two
pre-Pinedale lake sediment sequences in and near Yellowstone National Park.
Quaternary Research 10, 226–240.
Brown, P.M., Kline, D., Nash, S.E., 2014. Identification and dendrochronology of wood
found at the Ziegler Reservoir fossil site, Snowmass Village, Colorado. Quaternary Research. http://dx.doi.org/10.1016/j.yqres.2014.02.006 (in this volume).
Bryant, B., Martin, P.L., 1988. The geologic story of the Aspen region. U.S. Geological
Survey Bulletin 1603, 1–53.
CAPE Last Interglacial Project Members, 2006. Last interglacial arctic warmth confirms
polar amplification of climate change. Quaternary Science Reviews 25, 1383–1400.
Carrara, P.E., Mode, W.N., Rubin, M., 1984. Deglaciation and postglacial timberline in the
San Juan Mountains, Colorado. Quaternary Research 21, 42–55.
Please cite this article as: Anderson, R.S., et al., High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as
determined from pollen analysis, Quaternary Research (2014), http://dx.doi.org/10.1016/j.yqres.2014.03.005
R.S. Anderson et al. / Quaternary Research xxx (2014) xxx–xxx
Davis, O.K., 1998. Palynological evidence for vegetation cycles in a 1.5 million year pollen
record from the Great Salt Lake, Utah, USA. Palaeogeography, Palaeoclimatology,
Palaeoecology 138, 175–185.
Davis, O.K., Moutoux, T.E., 1998. Tertiary and Quaternary vegetation history of the Great
Salt Lake, Utah, USA. Journal of Paleolimnology 19, 417–427.
Elias, S.A., 2014. Environmental interpretation of fossil insect assemblages from MIS 5 at
Ziegler Reservoir, Snowmass Village, Colorado. Quaternary Research. http://dx.doi.
org/10.1016/j.yqres.2014.01.005 (in this volume).
Faegri, K., Iversen, J., 1989. Textbook of Pollen Analysis. Wiley, New York.
Fall, P.L., 1992. Pollen accumulation in a montane region of Colorado, USA: a comparison
of moss polsters, atmospheric traps and natural basins. Review of Palaeobotany and
Palynology 72, 169–197.
Fall, P.L., 1997. Timberline fluctuations and late Quaternary paleoclimates in the Southern
Rocky Mountains, Colorado. Geological Society of America Bulletin 109, 1306–1320.
Fawcett, P.J., Werne, J.P., Anderson, R.S., Heikoop, J.M., Brown, E.T., Berke, M.A., Smith, S.,
Goff, F., Hurley, L., Cisneros-Dozal, L.M., Schouten, S., Sinninghe Damsté, J.S., Huang,
Y., Toney, J., Fessenden, J., WoldeGabriel, G., Atudorei, V., Geissman, J.W., Allen, C.D.,
2011. Extended megadroughts in the southwestern United States during Pleistocene
interglacials. Nature 470, 518–521.
Fisher, D.C., Cherney, M.D., Newton, C., Graham, R.W., Rountrey, A.N., Calamari, Z.T.,
Stucky, R., Lucking, C., Petrie, L., 2014. Taxonomy and tusk growth analyses of Ziegler
Reservoir proboscideans. Quaternary Research (in this volume).
Haskett, D.R., Porinchu, D.F., 2014. A quantitative midge-based reconstruction of mean
July air temperature from a high-elevation site in central Colorado for MIS 6 and 5.
Quaternary Research (in this volume).
Heusser, L.E., 1995. Pollen stratigraphy and paleoecologic interpretation of the 160-K.Y.
record from Santa Barbara Basin, Hole 893A. In: Kennett, J.P., Baldauf, J.G., Lyle, M.
(Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, vol. 146
(pt. 2), pp. 265–279.
Heusser, L.E., 1998. Direct correlation of millennial-scale changes in western North
American vegetation and climate with changes in the California Current system
over the past 60 kyr. Paleoceanography 13, 252–262.
Heusser, L.E., 2000. Rapid oscillations in western North America vegetation and climate
during oxygen isotope stage 5 inferred from pollen data from Santa Barbara Basin
(Hole 893A). Palaeogeography, Palaeoclimatology, Palaeoecology 161, 407–421.
Heusser, C.J., Heusser, L.E., 1990. Long continental pollen sequence from Washington
State (USA): correlation of upper levels with marine pollen-oxygen isotope stratigraphy through substage 5e. Palaeogeography, Palaeoclimatology, Palaeoecology 79,
63–71.
Jiménez-Moreno, G., Anderson, R.S., 2012. Pollen and macrofossil evidence of late Pleistocene and Holocene treeline fluctuations from an alpine lake in Colorado, USA. The
Holocene 23, 68–77.
Jiménez-Moreno, G., Anderson, R.S., Fawcett, P.J., 2007. Orbital- and millennial-scale
vegetation and climate changes of the past 225 ka from Bear Lake, Utah-Idaho
(USA). Quaternary Science Reviews 26, 1713–1724.
Jiménez-Moreno, G., Fawcett, P.J., Anderson, R.S., 2008. Millennial- and centennial-scale
vegetation and climate changes during the late Pleistocene and Holocene from
northern New Mexico (USA). Quaternary Science Reviews 27, 1442–1452.
Jiménez-Moreno, G., Anderson, R.S., Atudorei, V., Toney, J.L., 2011. A high-resolution
record of vegetation, climate, and fire regimes in the mixed conifer forest of northern
Colorado (USA). Geological Society of America Bulletin 123, 240–254.
Johnsen, S.J., Dahl-Jensen, D., Gundestrup, N., Steffensen, J.P., Clausen, H.B., Miller, H.,
Masson-Delmotte, V., Sveinbjönrsdottir, A.E., White, J., 2001. Oxygen isotope and
palaeotemperature records from six Greenland ice-core stations: Camp Century,
Dye-3, GRIP, GISP2, Renland and NorthGRIP. Journal of Quaternary Science 16,
299–307.
Johnson, K.R., Miller, I.M., Pigati, J.S., 2014. Snowmastodon Project Science Team, 2014.
The Snowmastodon Project. Quaternary Research. http://dx.doi.org/10.1016/j.yqres.
2013.12.010 (in this volume).
Kukla, G.J., Bender, M.L., de Beaulieu, J.-L., Bond, G., Broecker, W.S., Cleveringa, P., Gavin, J.
E., Herbert, T.D., Imbrie, J., Jouzel, J., Keigwin, L.D., Knudsen, K.-L., McManus, J.F.,
11
Merkt, J., Muhs, D.R., Müller, H., Poore, R.Z., Porter, S.C., Seret, G., Shackleton, N.J.,
Turner, C., Tzedakis, P.C., Winograd, I.J., 2002. Last interglacial climates. Quaternary
Research 58, 2–13.
Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A.C.M., Levrard, B., 2004. A long
term numerical solution for the insolation quantities of the Earth. Astronomy and
Astrophysics 428, 261–285.
Langenheim, J.H., 1962. Vegetation and environmental patterns in the crested Butte Area,
Gunnison County, Colorado. Ecological Monographs 32 (3), 249–285.
Legg, T.E., Baker, R.G., 1980. Palynology of Pinedale sediments, Devlins Park, Boulder
County, Colorado. Arctic and Alpine Research 12, 319–333.
Lisiecki, L.E., Raymo, M.E., 2005. A Pliocene–Pleistocene stack of 57 globally distributed
benthic δ18O records. Paleoceanography 20, PA1003.
Litwin, R.J., Adam, D.P., Frederiksen, N.O., Woolfenden, W.B., 1997. An 800,000-year pollen
record from Owens Lake, California; preliminary analyses. GSA Special Papers 317,
127–142.
Mahan, S.A., Gray, H.J., Pigati, J.S., Wilson, J., Lifton, N.A., Paces, J.B., Blaauw, M., 2014. A
geochronologic framework for the Ziegler Reservoir fossil site, Snowmass Village,
Colorado. Quaternary Research (in this volume).
Markgraf, V., Scott, L., 1981. Lower timberline in central Colorado during the past
15,000 yr. Geology 9, 231–234.
Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore Jr., T.C., Shackleton, N.J., 1987. Age
dating and the orbital theory of the Ice Ages: development of a high-resolution 0 to
300,000-year chronostratigraphy. Quaternary Research 27, 1–29.
Miller, D.M., Jackson, S.T., Miller, I.M., 2014. Biogeography of Late Pleistocene conifer species from the Ziegler Reservoir fossil site, Snowmass Village, Colorado. Quaternary
Research (in this volume).
Nelson, A.R., Millington, A.C., Andrews, J.T., Nichols, H., 1979. Radiocarbon-dated upper
Pleistocene glacial sequence, Fraser Valley, Colorado Front Range. Geology 7,
410–414.
Ogden, T., 1979. Geologic map of Colorado, U.S. Geological Survey Geologic Map, 1:
500,000 scale.
Pierce, K.L., 2004. Pleistocene glaciations of the Rocky Mountains. In: Gillespie, A.R.,
Porter, S.C., Atwater, B.F. (Eds.), The Quaternary Period in the United States. Elsevier,
Amsterdam, pp. 63–76.
Pigati, J.S., Miller, I.M., Johnson, K.R., Honke, J.S., Carrara, P.E., Muhs, D.R., Skipp, G.,
Leonard, E.M., Plummer, M.A., Bryant, B., 2014. Geologic setting and stratigraphy of
the Ziegler Reservoir fossil site, Snowmass Village, Colorado. Quaternary Research.
http://dx.doi.org/10.1016/j.yqres.2013.12.011 (in this volume).
Reasoner, M.A., Jodry, M.A., 2000. Rapid respnse of alpine timberline vegetation to the
Younger Dryas climate oscillation in the Colorado Rocky Mountains, USA. Geology
28, 51–54.
Sears, P.B., Clisby, K.H., 1952. Two long climate records. Science 116, 176–178.
Sharpe, S.E., Bright, J., 2014. A high-elevation MIS 5 hydrologic record using mollusks and
ostracodes from Snowmass Village, Colorado, USA. Quaternary Research. http://dx.
doi.org/10.1016/j.yqres.2014.01.014 (in this volume).
Strickland, L.E., Baker, R.G., Thompson, R.S., Miller, D.M., 2014. Last interglacial plant
macrofossils and climates from Ziegler Reservoir, Snowmass Village, Colorado.
Quaternary Research (in this volume).
Toney, J.L., Anderson, R.S., 2006. A postglacial paleoecological record from the San Juan
Mountains of Colorado USA: fire, climate and vegetation history. The Holocene 16,
505–517.
Weber, W.A., Wittmann, R.C., 1996. Colorado Flora: Western Slope. University of Colorado
Press, Boulder.
Whitlock, C., Sarna-Wojcicki, A.M., Bartlein, P.J., Nickmann, R.J., 2000. Environmental history and tephrostratigraphy at Carp Lake, southern Columbia Basin, Washington,
USA. Palaeogeography, Palaeoclimatology, Palaeoecology 155, 7–29.
Woolfenden, W.B., 2003. A 180,000-year pollen record from Owens Lake, CA: terrestrial
vegetation change on orbital scales. Quaternary Research 59, 430–444.
Please cite this article as: Anderson, R.S., et al., High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as
determined from pollen analysis, Quaternary Research (2014), http://dx.doi.org/10.1016/j.yqres.2014.03.005